Aside from the nasal bone of the skull, the nose is formed by four plates of cartilage. Two form the the central mass of the nose, and two form the base and the wings that cover the nostrils. Each of these wings is called the ala (ALE-uh).
The lateral cartilages are triangular and curl over to meet each other at the crest of the nose. Each alar cartilage could be conceived as a faceted, oval ring; these meet at the base and form the anterior part of the nostrils. An alar lobe - consisting of fat - attaches to each alar cartilage and the surface of the face, forming the posterior part of the nostrils. The curved break around each nostril is called the alar furrow. The central part on the bottom is considered to be the septum, although the septum rises through the whole nose to divide it into two chambers.
On a one-bump nose, the prominence is where the cartilages meet the nasal bone. On a two-bump nose, such as the author's, the second prominence is at the bottom of the lateral cartilages.
It is a grievous sin of portraiture, and a common one of beginners, to draw the nostrils. The nostrils are hollows formed by the masses listed above, and those masses must be dealt with as dimensional shapes in order for any spaces within them to be convincing. The artist is advised to forget about the existence of nostrils, and let them be formed by rendering the surrounding alar masses. The reader is referred to "All Forms on the Body are Convex."